W0472

Structure of Staphylococcal Protease Chaperone/Inhibitor. C. Kent Brown¹, Zu-Yi Gu¹, Nicholas Nickerson², Martin J. McGavin², Douglas H. Ohlendorf¹, & Cathleen A. Earhart¹, ¹Dept. of Biochemistry, Molecular Biology and Biophysics, Univ. of Minnesota, Minneapolis, MN, ²Dept of Lab. Medicine and Pathobiology, Sunnybrook and Women’s College HSC, Univ. of Toronto, ONT Canada

The structure of the sspC gene product from Staphylococcus aureus has been solved to high resolution (1.8Å; R=21.4%, Rfree=24.8%). SspC encodes a 12.9 kDa protein that is a key player in maturation of the 2 major secreted proteases from S. aureus. Besides SspC, the ssp operon encodes a serine protease (endo-GluC, sspA) and a cysteine protease (endo-ArgC, sspB). Pro-SspA is believed to be cleaved by aureolysin; pro-SspB is cleaved by SspA. SspC is shown to form a stable complex with mature SspB, and inhibits its activity in a direct 1:1 ratio. When sspC is deleted from the ssp operon, SspA and SspB activity cannot be detected in the culture supernatant, suggesting that SspC in addition to functioning as a specific inhibitor of SspB, is also required for efficient secretion and/or maturation of SspA and SspB.

SspC folds into a 8 β-strand barrel of mixed topology plus 2 short helices at the top of the barrel. SspC shows significant shape homology to metalloprotease inhibitors from Erwinia chrysanthemi and Pseudomonas aeruginosa, despite altered topology of the first 3 strands and a total lack of sequence homology. The structure of SspC and a model of SspB are being examined in light of structures of proteases with inhibitors and pro-domains to gain insight into function.